Shape Measurement by Means of Phase Retrieval using a Spatial Light Modulator

We present a novel approach to investigate the shape of a diffusely reflecting technical object. It is based on a combination of a multiple-illumination contouring procedure and phase retrieval from a set of intensity measurements. Special consideration is given to the design of the experimental configuration for phase retrieval and the iterative algorithm to extract the 3D phase map. It is mainly based on a phase-only spatial light modulator (SLM) in the Fourier domain of a 4f-imaging system. The SLM is used to modulate the light incident in the Fourier plane with the transfer function of propagation. Thus, a set of consecutive intensity measurements of the wave field scattered by the investigated object in various propagation states can be realized in a common recording plane. In contrast to already existing methods, no mechanical adjustment is required during the recording process and thus the measuring time is considerably reduced. The method is applied to investigate the shape of micro-objects obtained from a metalforming process. Finally, the experimental results are compared to those provided by a standard interferometric contouring procedure.